In theory they shouldn't ever be available via any other method as even right now on the official site's FAQ pages they describe the minions as "Fan Festival-exclusive minions".

If they later release them another way they risk a lot of people being very annoyed at them and risk future sales (and I'm sure more than one person would argue they're due a refund...)

So for safety's sake, assume that after August 31st the minions won't be available again.

The attire though, doesn't specifically say exclusive to the 3 events. So that possibly might be released later, bad taste if they do, but I see it as possible? Wouldn't count on it though. You only need to purchase one event to get the outfit you need though.